1. Field of the Invention
Embodiments of the present invention relate to methods of silicon layer deposition and, more particularly, to methods of silicon layer formation using cyclical deposition techniques for active matrix liquid crystal display (AMLCD) applications.
2. Description of the Background Art
Active matrix liquid crystal displays have eliminated many problems associated with passive displays. For example, the fabrication of active matrix liquid crystal displays have enabled display screens to achieve greater brightness, enhanced readability, a greater variety of color shades, and broader viewing angles compared to displays that employ other technologies. Active matrix liquid crystal displays have therefore become the display technology of choice for numerous applications including computer monitors, television screens, camera displays, avionics displays, as well as numerous other applications.
Active matrix liquid crystal displays generally comprise an array of picture elements called pixels. An electronic switch is associated with each pixel in the display to control the operation thereof. Various electronic switches such as, for example, thin film transistors, and organic light emitting diodes (OLED), among others have been investigated to control pixel operation. Thin film transistors, in particular, offer a high degree of design flexibility and device performance.
Thin film transistors are generally formed on large area substrates having a high degree of optical transparency such as, for example, glass. FIG. 1 depicts a cross-sectional schematic view of a thin film transistor 22 being a type that has a bottom gate structure. The thin film transistor 22 includes a glass substrate 1 having an underlayer 2 formed on the surface thereof. A gate is formed on the underlayer 2. The gate comprises a gate metal layer 4 and a gate dielectric 8. The gate controls the movement of charge carriers in the transistor. A gate dielectric 8 formed over the gate metal layer 4 electrically isolates the gate from semiconductor layers 10, 14a, 14b, formed thereon, each of which may function to provide charge carriers to the transistor. A source region 18a of the transistor is formed on semiconductor layer 14a and a drain region 18b of the transistor is formed on semiconductor layer 14b. Finally, a passivation layer 20 encapsulates the thin film transistor 22 to protect it from environmental hazards such as moisture and oxygen.
Many thin film transistors use silicon for the semiconductor layers 10, 14a, 14b. Amorphous silicon, in particular, is widely employed because it is easy to deposit at low temperatures using techniques such as, for example, plasma enhanced chemical vapor deposition (PECVD). Unfortunately, it is difficult to deposit amorphous silicon layers that are continuous (e.g., without gaps or voids) using PECVD techniques. Amorphous silicon layers also tend to have a lower electron mobility. A low electron mobility for the amorphous silicon may limit the speed of transistors formed therefrom.
As such, polycrystalline silicon has been actively investigated as a substitute for amorphous silicon in thin film transistors. Polycrystalline silicon also has an electron mobility several orders of magnitude greater than that of amorphous silicon, which allows for the formation of fast-switching thin film transistors.
Unfortunately, conventional plasma enhanced chemical vapor deposition (PECVD) techniques used to form polycrystalline silicon tend to be high temperature processes. High deposition temperatures may not be compatible with the glass substrates upon which the thin film transistors are formed, since the glass tends to soften and become dimensionally unstable.
To circumnavigate this problem, some transistor fabrication processes form polycrystalline silicon by first depositing a layer of amorphous silicon at relatively low temperatures and then annealing the layer using a laser or a furnace to convert the amorphous silicon to polycrystalline silicon. While the electron mobility is higher for polycrystalline silicon films formed using an annealing process than for an amorphous silicon film, the electron mobility of such films is still lower than the electron mobility for polycrystalline silicon films directly deposited on a substrate from a plasma enhanced chemical vapor deposition (PECVD) process. Furthermore, annealing requires an additional step, thereby reducing the process throughput of thin film transistor fabrication processes.
Therefore, a need exists to develop a method of forming silicon layers for use in thin film transistors.